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Cellular elements
2 categories
1. Nerve cells/neurons- concerned with information processing and signalling
2. Glial cells- supporting role 100 billion neurons; maybe more glial
cells
If we took 1 second to count one neuron, counting 100 billion neurons would take more than 3000 years
All neurons are variations on the same theme
Convey information by combined electrical and chemical signaling mechanisms
Electrical signals- rapid transmission of information from one part of neuron to another
Chemical messengers carry information between neurons
Anatomically specialized areas for collecting, integrating, conducting and transmitting information
Components of neuron
Cell body= soma/perikaryon [karyon=nucleus]
Supports metabolic and synthetic needs of the rest of the neuron
Processes of neurons
Dendrites –series of branching tapering processes
Receive information from other neurons via synaptic contacts/synapses
Axon
One long cylindrical process Conducts information away from cell
body Gives rise to a series of terminal
branches, forming synapses on other neurons
Neurons are anatomically and functionally polarized, with electrical signals travelling in only one direction under ordinary physiologic circumstances
Anatomical classification
Depends on number of processes Multipolar- vast majority-multiple
dendrites Bipolar- 2 dendrites Pseudounipolar -
Unipolar neurons are actually pseudounipolar
They start out as bipolar, but during development, cell body expands asymmetrically, leaving behind a stalk from which both processes emerge
Located in dorsal root and cranial nerve ganglia
Formation of a pseudounipolar neuron
Motor neurons and interneurons are multipolar
Bipolar neurons are located in retina and CN VIII ganglia
Functional classification
Sensory neurons
Directly sensitive to various stimuli [e.g. touch or temperature] or receive direct connections from non-neuronal receptor cells
Sensory neurons
Their processes are included in somatic and visceral afferents
Somatic afferents convey pain, temperature, touch, pressure, proprioception
Visceral afferents convey pain and other sensations from mucous membrane, glands and blood vessels
Most sensory and motor neurons live partly in CNS and partly in PNS
The words sensory and motor are often used in a broader sense to refer to cells and axons carrying information related to sensory stimuli and the responses generated
Motor neurons
Convey impulses from CNS/ganglia to effector cells
Their processes are included in efferent nerve fibres
Somatic efferents → skeletal muscles Visceral efferents → smooth muscle,
heart, glands
Interneurons
≥ 99% of all neurons Form a communicating and integrating
network between sensory and motor neurons
Local interneurons have all there processes confined to a single area od CNS
Probably not more than 20 million sensory fibres in all of spinal cord and cranial nerves combined
No more than few million motor neurons
Projection neurons
Have long axons connecting different areas, such as a neuron in cerebral cortex whose axon reaches spinal cord
Strictly speaking, human nervous system is almost entirely composed of interneurons and projection neurons
More than 99% are interneurons or projection neurons
GRAY MATTER AND WHITE MATTER
CNS is easily divisible into gray matter and white matter
Gray matter- preponderance of cell bodies and dendrites. In life it is pinkish gray due to abundant blood supply
White matter
preponderance of axons; many of whom have myelin sheath
Myelin sheath is mostly lipid- hence the white appearance
Nuclei
Specific areas of gray matter in CNS whose neurons are functionally related- similar areas in PNS are called ganglia
Cortex
An area where gray matter forms a layered surface covering some part of CNS
Subdivisions of white matter[collections of axons] Variety of names in CNS- fasciculus,
funiculus, lemniscus, peduncle- most commonly tracts
Collections of axons in PNS are called nerves
Fasciculus = ‘little bundle’ Funiculus = ‘string’ Lemniscus = ‘ribbon’- tracts flattened out
in cross section Peduncle= ‘little foot’- site where tracts
funnel down into a compact bundle
Features of a neuron
Synthesizes
1. neuronal enzymes,
2. structural proteins,
3. membrane components,
4. organelles and
5. some of its chemical messengers [neurotrnsmitters]
Nucleus; large, pale staining with dispersed chromatin
Abundant RER, free ribosomes, stacks of Golgi apparatus
Many mitochondris
Nissl bodies/Nissl substance- ribosomes, stained intensely with basic dyes, appear as clumps- prominent in large neurons
Many mitochondria
Cytoskeleton composed of microtubules, neurofilaments [aggregates of these are called neurofibrils] and microfilaments
Dendrites
Tapered extensions of neuronal body Collectively provide a great increase in
surface area available for synaptic inputs
In spinal cord, dendritic surface area may be 30 or more times that of cell body
Dendrites of many neurons are studded with small protuberances called dendritic spines
These are preferred sites of some synaptic contacts
Dendrites and dendritic spines
Axons
Different from dendrites Cylindrical process- arises abruptly from
an axon hillock on one side of neuronal body
Initial segment has bundles of microtubules, neurofilaments and mitochondria- no Nissl substance
It is most electrically excitable part of a neuron
Beyond initial segment, many axons are encased in spiral wrapping of a membrane called myelin sheath- greatly increases speed of propagation of electrical impulses
Transport of macromolecules and organelles synthesized by cell body occurs away from soma[anterograde] and towards it [retrograde]
It can be slow or fast Microtubules act as ‘railroad tracks’ for
fast transport
Many peripheral nerves are myelinated- resemble a string of sausages
Each link of sausage corresponds to a length of axon wrapped in myelin with adjacent links separated by a gap in myelin
At this site axon is separated from extracellular space only by fingerlike processes from Schwann cells
Myelin sheath between 2 nodes is called internode- formed by s single Schwann cell
Most of smaller axons in peripheral nerves are unmyelinated- slow conductors of electrical signals
Glial cells
Glia =Gr. Glue Cells are so named because they fill up
most of the spaces between neurons- appear to hold them in place
Some do provide structural support Play a wide variety of additional roles
Neuroglial cells, collectively known as the neuroglia or simply as glia, have important ancillary functions.
The neuroglial cells of the normal CNS are astrocytes, oligodendrocytes, ependymal cells (derived from neural tube ectoderm), and microglia (derived from mesoderm)
Astrocytes occur throughout the brain and spinal cord
Oligodendrocytes produce myelin and are also found next to the cell bodies of some neurons.
Microglial cells become phagocytes when local injury or inflammation is present.
The neuroglial cells of the peripheral nervous system are Schwann cells in nerves and satellite cells in ganglia.
Synapse
Neurons, neuropil, and the common glial cells of the CNS
Neurons, neuropil, and the common glial cells of the CNS
Supporting cells
Glia/neuroglia in CNS Schwann cells and supporting cells in
PNS Schwann cells surround neurites,
isolating them from adjacent cells and extracellular matrix
In PNS ganglia, supporting cells are Satellite cells- surround nerve cell bodies [nucleus containing part]- analogous to Schwann cells
In development, glial cells serve as scaffolding that directs neuronal migration to appropriate sites
Functions
Physical support for neurites Electrical insulation for nerve cell bodies
and processes Metabolic exchange pathway between
vascular system and neuronsS
Oligodendrocytes
Processes extend around several axons in CNS
Predominant glial cell in white matter
Astrocytes
Form a network of cells within CNS Communicate with neurons- modulate
and support their activities Fibrous astrocytes- few, long processes-
found in white matter Protoplasmic astrocytes- many short
branched processes- found in gray matter
Important role in proper formation of CNS in fetal and embryonic development
Control ionic environment of neurons Form scar tissue in CNS damage
Perivascular feet contribute to blood- brain barrier Regulate vasodilatation Regulate transfer of oxygen, ions and
other substances from blood to neurons
Ependymal cells
Low cuboidal/columnar Line ventricles of brain and central canal
of spinal cord In some places they are ciliated to
facilitate movement of CSF- in others they have long microvilli
Microglia
5% of all glial cells Part of MPS- monocyte precursors Proliferate and become actively
phagocytic in regions of injury and disease
Remove debris of apoptotic cells during CNS development
Synapse
Types of synapses
Nerve injury/regeneration Nerve cells, unlike neuroglial cells,
cannot proliferate but can regenerate their axons, located in the PNS